Movable crosshead loading device



March 18, 1969 G. s. BURR 3,433,061

MOVABLE CROSSHEAD LOADING DEVICE Filed June 2'7, 1966 AMPLIFIER i2 70 eaCOMMAND SIGNAL COMPARATOR SOURCE AMPLIFIER I v AMPLIFIER AMPLIDYNE lUnited States Patent 3 Claims ABSTRACT OF THE DISCLOSURE Materialstesting machine having a hydraulic ram between its movable crosshead andone of two grips, a control system for actuating the ram to adjust acondition of the specimen, and a control system to adjust the crossheadincluding a sensor for directly sensing the relative positions of theram and the crosshead.

This invention relates to control systems for materials testing machineshaving movable crossheads.

It is a primary object of this invention to provide, in such a machinehaving its crosshead coupled to the movable grip through an hydraulicram, for a highly accurate and reliable, yet simple system forcoordinating ram and crosshead position.

The invention features, in a materials testing machine, a movablecrosshead, a pair of grips for holding a specimen to be tested, a drivemechanism for moving the crosshead and a hydraulic ram. The hydraulicram has a first portion mounted for movement with the crosshead and asecond portion coupled to one of the grips and hydraulically coupled tothe first portion for movement relative thereto parallel to thecrosshead. The ram has a stroke substantially shorter than the overalllength of travel of the crosshead. A control system actuates the ram inresponse to a deviation between the actual and desired position of thegrip coupled to the second portion of the ram A, and controls the drivemechanism and ram to restore, after actuation, the relative positioningof the crosshead and second portion of the ram. The control systemincludes a sensor mounted to directly sense the relative positioning ofthe crosshead and second portion of the ram.

Other objects, features, and advantages will appear from the followingdescription of a preferred embodiment of the invention, taken togetherwith the attached drawing thereof which is a diagram of a materialstesting machine with a hydraulic ram and the control system of thisinvention.

There is shown in the drawing a portion of a materials testing machine10 having a frame 12; crosshead 14 driven by motor 26 through screws 16and 18 and gears 20, 22, and 24; and specimen grips 28 and 3t Mounted oncrosshead 14 is a hydraulic ram including a hydraulic cylinder 32 andpiston 34. Connecting rod 36, in fluid-tight, slidable engagement withaperture 38, conects grip 30 to piston 34. Rod 40, in fluid-tight,slidable engagement with aperture 42, connects to piston 34 and extendsthrough sleeve 44 in crosshead 14.

Chamber 46 has compartments 48 and 59 on opposite sides of piston 34.Passages 52 and 54 connect compartments 48 and 50, respectively, toelectrically controlled valve 56 which selectively applies a source ofhydraulic pressure to the hydraulic fluid 58 in said compartments. Tube60 connects valve 56 to a source of hydraulic pressure (not shown).

A load cell 62 senses the load being applied to the specimen 64 by themachine 10 and produces a negative voltage which is introduced throughamplifier 66 to an input of comparator 68. Command signal source 70,

ice

which may be a manually operated potentiometer circuit, provides apositive voltage indicative of the desired load to be applied tocomparator 68. The output of comparator 68 is delivered throughamplifier 72 to valve 56.

Fastened to crosshead 14 is a linear variable differential transformer(LVDT) 74 which senses the axial move ment of rod 40 in relation tocrosshead 14. The output of LVDT 74 is a positive voltage when piston 34is displaced from its centered position in cylinder 32 toward crosshead14; a negative voltage results when piston 34 moves from its centeredposition towards grip 30. In either case the output of LVDT 74 controls,through amplifier 76, the output of amplidyne 78, which in turn controlsmotor 26. A positive output voltage from ILVDT 74 causes motor 26 tomove crosshead 14 downwardly; a negative voltage to move it upwardly.

In operation, with the machine in a stable state, i.e., with zero outputfrom comparator 68, a call for an increased load by manipulation ofcommand signal source 70 instantaneously causes a positive voltage toappear at the output of comparator 68. After being amplified inamplifier 72, this positive voltage operates valve 56 connecting channel52 to the source of hydraulic pressure through tube 60, therebyincreasing the pressure in compartment 48 and driving piston 34downwardly.

The downward movement of piston 34 is sensed by LVDT 74 and causes it totransmit a positive signal representative of the displacement of piston34 to amplifier 76 and then to amplidyne 78 where it controls thepolarity and magnitude of the power to motor 26. Since the voltage fromLVDT 74 is now positive, motor 26 is driven to move crosshead 14downwardly until piston 34 is once again centered in cylinder 32.

In a similar manner, a call for a decreased load causes upward movementof piston 34 and corresponding coordination of the crosshead position.

The subject matter discloses above and relating to the use of ahydraulic ram with a crosshead in a materials testing machine is thejoint invention of John Linnartz Preston and George Sterling Burr and isthe subject of a separate patent application filed the same day as thisapplication.

Other embodiments (e.g. in position control operation) will occur tothose skilled in the art and are within the following claims.

I claim:

1. A materials testing machine comprising:

a movable crosshead,

a pair of grips for holding a specimen to be tested,

a drive mechanism for moving said crosshead,

an hydraulic ram having a first portion mounted for movement with saidcrosshead and a second portion coupled to one of said grips andhydraulically coupled to said first portion for movement relativethereto parallel to said crosshead,

a support for the other of said grips,

a source of hydraulic fluid for said ram,

a sensor for sensing a condition of said specimen,

a first control system responsive to said sensor for actuating said ramin response to a deviation between the actual and desired value of saidcondition, and

a second control system for actuating said drive mechanism to restore,subsequent to said actuation of said ram, the relative positioning ofsaid crosshead and said second portion of said ram, said control systemincluding a second sensor mounted to directly sense the relativepositioning of said crosshead and said second portion of said ram.

2. The machine of claim 1 wherein said second sensor commands said drivemechanism for said crosshead.

3 4- 3. The machine of claim 2 in which said second sensor Structural &Materials Testing Systems, pub. MTS is an LVDT, and said first controlsystem further com- Division, Research Incorporated, dated August 1964.prises: Tinius Talks, vol. 17, no. 1, published by Tinius Olsen anamplifier, responsive to the output of said LVDT, Testing Mach. Co.,November 1965, pages 1, 6, and 7.

and 5 an amplidyne, responsive to the output of said am- JAMES J, GILL,Primary Examiner.

plifier, for controlling said drive mechanism. JERRY W MYRACLE AssistantExaminer References Cited US Cl XR.

A New System of Weighing Machinery, published by the Yale & Towne Mfg.Co., pages 225 and 253-255, published June 1884.

